In a wireless power feeding system for wirelessly feeding power from a power transmission device to a power reception device by magnetic-field coupling, the power transmission device includes a power transmission coil, and the power reception device includes a power reception coil. One example of that wireless power feeding system is illustrated in Japanese Unexamined Patent Application Publication No. 2014-23323. In that illustrated wireless power feeding system, the power reception device transmits a signal to the power transmission device, and the power transmission device grasps the state of the power reception device.
In the wireless power feeding system illustrated in Japanese Unexamined Patent Application Publication No. 2014-23323, at the time of transmission of a signal from the power reception device to the power transmission device, power transport from the power transmission device to the power reception device is interrupted, power transport timing at which power is transported from the power transmission device to the power reception device and signal transmission timing at which a signal is transmitted from the power reception device to the power transmission device are alternately repeated, and power is transported from the power transmission device to the power reception device and signals are transmitted from the power reception device to the power transmission device.
In this wireless power feeding system, the power transmission device includes a detection circuit for detecting impedance on the load side, the detection circuit detects a change in the load impedance of the power transmission coil during the power transport and detects a signal transmitted to the side where the power transmission device is disposed. The power transmission device cannot grasp timing at which a signal is sent by the power reception device. Thus, the power transmission device causes the detection circuit to operate constantly so as not to fail to detect signals from the power reception device. This raises a problem that power consumption of the side where the power transmission device is disposed is increased.